This application is based on Korean Patent Application No. 2001-66536, filed in the Korean Intellectual Property Office on Oct. 27, 2001, the disclosure of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium-sulfur battery and a method of preparing the same, and more particularly, to a positive active material for a rechargeable lithium-sulfur battery having an improved discharge voltage at high C-rates, and improved power and cycle-life characteristics, and a method of preparing the same.
2. Description of the Related Art
A rechargeable lithium-sulfur battery includes a positive active material including a sulfur-based compound having a sulfur-sulfur bond, and a negative active material including lithium metal or a carbonaceous material. The sulfur-based compound may be elemental sulfur (S8) or an organo sulfur. The carbonaceous material is a material in which intercalation chemistry occurs, examples of which include graphite intercalation compounds, carbonaceous materials, and carbonaceous materials inserted with lithium. Upon discharge (electrochemical reduction), a sulfur-sulfur bond breaks, which results in a decrease in the oxidation number of S. Upon recharging (electrochemical oxidation), a sulfur-sulfur bond forms, which leads to an increase in the oxidation number of S.
Although such a rechargeable lithium-sulfur battery has a low discharge voltage (on the order of 2 V), a lithium-sulfur battery has been proposed as a next generation rechargeable battery following a lithium-ion battery and a lithium polymer battery since it has excellent stability, low cost, and high charge and discharge capacities. However, the lithium-sulfur battery has not yet been commercialized since it has low electrochemical activity. The low electrochemical activity results from the fact that S8 is electrochemically inactive and that a passivation layer is formed on the surface of lithium metal, which results in a battery having a poor discharge voltage at a high C-rate and poor cycle-life characteristics.
In order to improve the electrochemical activity, it is proposed to add a certain material to the composition of the positive active material. The certain material is able to increase the absorption property of polysulfide.
By adding an absorption agent to a positive active material active mass, the mass is prevented from separating from a current collector. For the absorption agent, Japanese Patent Laid-open No. Hei. 9-147868 (Jun. 6, 1997) discloses an active carbon fiber. U.S. Pat. No. 5,919,587 discloses a method that, using a transition metal calcogenide having a highly porous, fibrous, and ultra-fine sponge-like structure, embeds the positive active material therein or encapsulates the positive active material therewith. Further, International Patent Publication No. WO 99/33125 discloses a microparticle having a high absorbent strength that is coated on a positive electrode or that is added into a positive active material mass. International Patent Publication No. WO 99/33127 discloses that, by using a cationic polymer including a quaternary ammonium salt group, the polysulfide anion is maintained around the cationic polymer.
However, there are continuing demands for further improved positive active materials, and especially for improvements in the discharge voltage at a high C-rate and the cycle-life characteristics.
It is an object of the present invention to provide a positive active material for a rechargeable lithium-sulfur battery cell exhibiting excellent cycle-life characteristics.
It is another object of the present invention to provide a positive active material for a rechargeable lithium-sulfur battery cell exhibiting excellent discharge voltage characteristics at a high C-rate.
It is yet another object of the present invention to provide a positive active material for a rechargeable lithium-sulfur battery cell exhibiting excellent power characteristics.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
In order to achieve these and other objects, an embodiment of the present invention provides a positive active material for a rechargeable lithium-sulfur battery including a core of a sulfur compound and a surface-passivation layer of a coating element formed on the core.
According to an aspect of the invention, the surface-passivation layer is made of a coating-element-included compound selected from the group consisting of a coating-element-included hydroxide, a coating-element-included oxyhydroxide, a coating-element-included oxycarbonate, a coating-element-included hydroxycarbonate, and a mixture thereof.
According to another aspect of the invention, the coating element is Al, Si, or B.
According to yet another aspect of the invention, the coating element is B.
According to still another aspect of the invention, the coating-element-included compound includes hydrogen borate.
According to another embodiment of the invention, a method of preparing a positive active material for a rechargeable lithium-sulfur battery includes coating a sulfur compound with a coating liquid including a coating material source, and drying the coated sulfur compound.
According to a further aspect of the invention, the coating material source includes Al, Si, or B.
According to a still further aspect of the invention, the coating material source includes B.